As disclosed in, for example, JP2012-147671A corresponding to US2009/0002054A1, a constant current drive may be employed in a gate driver for driving a gate of a transistor such as an insulated-gate bipolar transistor (IGBT). Compared to a constant voltage drive, the constant current drive has an advantage that a switching loss at the time of the rising is small. As disclosed in, for example, JP10-32476A, a drive circuit may be provided with a clamp circuit for clamping a gate voltage to a predetermined voltage. For example, when an IGBT falls in an overcurrent state, the clamp circuit protects a gate voltage of the IGBT from increasing.
After a deep study, the present inventor has found out that using the constant current drive and the clamp circuit in combination in a gate driver may result in the following problem. Specifically, although a gate drive current decreases with an increase in a gate voltage in the constant voltage drive, a constant gate drive current flows regardless of an increase in a gate voltage in the constant current drive. For this reason, there is a trend that a gate drive current is larger in the constant current drive than in the constant voltage drive.
For example, the clamp circuit has devices including a Zener diode and a MOS transistor. When a gate voltage seeks to increase above a predetermined clamp voltage, the clamp circuit performs a clamp action that clamps the gate voltage to the clamp voltage by drawing a gate drive current to a ground through the devices. In the clamp action, the clamp circuit is required to draw a large amount of the gate drive current in the constant current drive compared to in the constant current drive. Accordingly, a loss in the gate driver increases.
A parasitic inductance component (hereinafter referred to as the “parasitic inductor”) derived from a wire and the like exists between the gate driver and the gate of the IGBT. As described above, since the gate drive current is larger in the constant current drive than in the constant voltage drive, a current flowing through the parasitic inductor in a normal state is large accordingly. Therefore, in an abnormal state such as an overcurrent state, the clamp circuit performs a current draw action that draws the entire large gate drive current to the ground. In this case, the current draw action may be excessively performed due to recovery characteristics of the MOS transistor and due to response delay in a gate control circuit. As a result, the current flowing through the parasitic inductor changes largely between a positive value and a negative value. It is noted that the current has a positive value when flowing toward the gate.
At this time, a surge occurs in the gate of the IGBT according to an inductance (L) of the parasitic inductor and a rate of change (di/dt) in the current flowing through the parasitic inductor. The surge is given by the following formula: “−Lx(di/dt)”. That is, as the current change rate is higher, the surge is larger. Accordingly, compared to in the constant voltage drive, the surge is large in the constant current drive where a relatively large current flows through the parasitic inductor in a normal state.
If the gate voltage increases due to the surge and exceeds a gate breakdown voltage of the IGBT, the IGBT may be broken or reduced in life. If the gate voltage further increases due to the surge, the IGBT is more fully ON (i.e., ON-resistance of the IGBT becomes smaller) despite the overcurrent state. Accordingly, its corrector current further increases, so that the IGBT may be broken.